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Experimental Nonmechanical Image Rotation to 20 Angles Using an Acousto-Optic Dove Prism



Y S. Im, E G. Paek, Joon Y. Choe, Tae K. Oh


An experimental non-mechanical image rotation using a pair of crossed acousto-optic beam deflectors and a polygon mirror is described.Image rotation is important for various image manipulation, processing and recognition. Most of proposed methods to date rely on digital electronic computers to change coordinates and so are inevitably slow. To avoid the problem, we have proposed a new ultrafast non-mechanical image rotation method using an acousto-optic dove prism (AODP). In the initial experiment, we could rotate an image to only two different directions using a pair of mirrors. In this paper, we describe an experimental result of image rotation to with a custom-made 20 facet polygon mirror to prove the concept of the image rotation to arbitrary angle.Briefly describing the non-mechanical image rotation system, light from a laser source is expanded and illuminates an input image. The light passing through the input is appropriately diffracted by the first two-dimensional acousto-optic beam deflector (2-D AOBD) that consists of a crossed Bragg cells oriented along the x and y directions. The diffracted light is then focused by a Fourier transform lens on the surface of one of the facets of an internal polygon mirror. Light reflected from the mirror facet is then diffracted by the second 2-D AOBD and forms a rotated version of the original image at the focal plane of the second Fourier lens. In order to save optical components and to make a system compact, a mirror is used just after the polygon mirror to fold the system.The 2-D AOBD for this application should have a wide frequency bandwidth to sweep along a broad range of angles. Bandwidth is directly related with the available number of different rotation angles and resolution of an image. Also, the AOBD's should have a wide input angular bandwidth not to cause spurious striation in the diffracted light beam. One should note that incoming beams onto the AOBD's are not collimated but instead focused with a wide angular bandwidth. Also, it should have low insertion loss, uniform beam pattern, etc.In order to keep the center of rotated images fixed, all the mirror facets of a polygon must be oriented precisely parallel to the optical axis. The angles between adjacent mirrors need to be aligned without leaving noticeable seams between mirrors. A special care must be taken to keep the back mirror exactly normal to the optical axis to permit retro-reflection. Also, the seam between the polygon mirror and the back mirror must be un-noticeable and clean so as not to cause any scattering. Also, internal mirror surfaces must be flat and clean without any dust or blemishes so as not to scatter light.In our experiment, light from a 10 mW HeNe laser is expanded and illuminates an input mask (resolution chart). As shown in Figure 1 (a), the polygon mirror has 20 facets in a half cylinder, forming an angle between adjacent mirror facets of around 171 degrees. Special care was taken to meet the requirements described above, especially the back mirror normal to each of the mirror facets to ensure retro-reflection.Figure 1 (b) demonstrates rotation of an image (resolution chart) to eight different angles within a few microseconds. These results clearly demonstrate the concept of the non-mechanical image rotation into a reasonable number of angles.In conclusion, we have demonstrated experimental results of an ultrafast non-mechanical image rotation.
Optical Engineering
No. 11


acousto-optic beam deflectors, electro-optic sensors, electro-optics, image processing, image recognition, non-mechanical image rotation


Im, Y. , Paek, E. , Choe, J. and Oh, T. (2000), Experimental Nonmechanical Image Rotation to 20 Angles Using an Acousto-Optic Dove Prism, Optical Engineering (Accessed October 4, 2023)
Created October 31, 2000, Updated October 12, 2021